Social cues may play an important role in entraining circadian rhythms by enabling animals to synchronize their behaviors (temporal synchronization) to achieve common goals, or to avoid each other (temporal partitioning) to lessen competition for a limited resource. Prolonged cohabitation may lead to robust effects on the rhythmicity of co-housed individuals, but a major problem has been that present methods of recording daily activity cannot distinguish the activity of individual animals housed together in the same cage. In addition, few studies have prospectively considered specific types of social interactions (e.g., fighting behaviors, dominance status) that are likely to be significant variables. In this fellowship application, I would like to address these issues using a novel technology (a custom-designed implanted "micrologger" that functions as a self-contained activity acquisition and storage unit) and new experimental paradigms. I present preliminary data in paired Syrian hamsters and laboratory mice to suggest that cohabitation results in long-lasting changes to the circadian system. In Aim 1, I ask if cohabitation of pairs of Syrian hamsters affects measures of circadian locomotor activity in constant darkness; if such cohabitation effects are due to masking or pacemaker entrainment and depend on the presence of a running wheel; and if such effects result in actual temporal partitioning when the co-housed pairs are then entrained to light/dark cycles. In Aim 2, I ask if cohabitation of pairs of laboratory mice alters their circadian activity rhythms in constant darkness; if such cohabitation modifies the activity of arrhythmic homozygous Clock mutants and their heterozygous or wild type cohabitants; and how these effects are reflected in patterns of gene expression in the suprachiasmatic nucleus (SCN) and other brain regions. In Aim 3, I ask if cohabitation of pairs of spiny mice (Acomys cahirinus and Acomys russatus), which exhibit temporal partitioning in nature, can reproduce such partitioning in the laboratory. I will determine the mechanism for this phenomenon (masking or pacemaker entrainment) and analyze patterns of SCN gene expression when A. russatus is active "diurnally". By investigating a variable (social interactions) that is common in nature, but often overlooked in the laboratory setting, these experiments will add a new dimension to our understanding of the circadian system as well as standard laboratory housing practices. Notably, humans suffer from circadian disorders, and our investigation of the effects of social cues on the circadian system could lead to the development of non-photic approaches to help these individuals entrain to a daily schedule or more easily shift to a new one, thereby reducing the physiological and psychological consequences of these disorders. [unreadable] [unreadable] [unreadable]